Spin-on-glass (SOG) is frequently used for gap fill and planarization of inter-level dielectrics (ILD) in multi-level metalization structures. It is a very suitable material for use in low-cost fabrication of IC circuits. Most commonly used SOG materials are of two basic types; an inorganic type of silicate based SOG and an organic type of siloxane based SOG. One of the commonly used organic type SOG materials is a silicon oxide based polysiloxane which is featured with radical groups replacing or attaching to oxygen atoms. Based on these two basic structures, the molecular weight, the viscosity and the desirable film properties of SOG can be modified and adjusted to suit the requirement of specific IC fabrication process.
SOG film is typically applied to a pre-deposited oxide surface as a liquid to fill gaps and steps on the substrate. Similar to the application method for photoresist films, a SOG material can be dispensed onto a wafer and spun with a rotational speed which determines the thickness of the SOG layer desired. After the film is evenly applied to the surface of the substrate, it is cured at a temperature of approximately 400.degree. C. and then etched back to obtain a smooth surface in preparation for a capping oxide layer on which a second interlevel metal may be patterned. The purpose of the etch-back step is to leave SOG between metal lines but not on top of the metal, while the capping oxide layer is used to seal and protect SOG during further fabrication processes. The siloxane based SOG material is capable of filling 0.15 micron gaps and therefore it can be used in 0.25 micron technology.
When fully cured, silicate SOG has similar properties like those of silicon dioxide. Silicate SOG does not absorb water in significant quantity and is thermally stable. However, one disadvantage of silicate SOG is the large volume shrinkage during curing. As a result, the silicate SOG retains high stress and cracks easily during curing and further handling. The cracking of the SOG layer can cause a serious contamination problem for the fabrication process. The problem can sometimes be avoided by the application of only a thin layer, i.e., 1000.about.2000 .ANG. of the silicate SOG material.
In the current SOG coating process, a solvent edge rinse and a solvent backside rinse process are utilized to remove unwanted SOG deposited on the wafer edge and on the backside of the wafer. This is shown in FIGS. 1.about.3. A semiconductor wafer 10 which has a flat side 12 is shown in FIG. 1. After a SOG coating process, a SOG layer 14 is blanket deposited on the top surface 16 of the wafer. The layer is deposited as a dielectric layer for insulating between metal lines. In order to process the wafer in subsequent fabrication steps, the wafer must be positioned in reaction chambers for various processes such as etching or deposition. In most of the process chambers, the wafer is positioned on a platform and held down on the edge by a wafer clamp. The function of the wafer clamp is to prevent the wafer from moving during the process when reactant gases or etching gases may be flowing into the reaction chamber. To enable the wafer clamp to function properly, the edge portion of the wafer of approximately 2.about.4 mm wide must be cleaned without any coated material. This edge area 22 on wafer 10 is shown in FIG. 1.
In present wafer fabrication technology, the SOG layer deposited at unintended areas of the wafer can be removed in two different processes. The first process is a solvent edge rinse which is shown in FIG. 2. In this process, wafer 10 is placed on a platform (not shown) and spun at a predetermined rotational speed along a spin axis 26. The rotational speed of the wafer can be suitably adjusted for each specific application depending on the thickness of the layer to be removed and the type of chemical solution used. As shown in FIG. 2, a chemical solution injector 28 is used to inject chemical solution 32 onto the top edge 34 of the wafer. The chemical solution 38 deflected from the edge 34 of the wafer hits the chamber wall 42 and drains to the bottom of the process chamber. The solvent edge rinse process is effective in removing a limited area, i.e., a width of 2.about.4 mm, on the top edge of the wafer of unwanted coating materials such as SOG or photoresist. However, as shown in FIG. 1, when wafer 10 is spun around its center on a rotational axis, the flat side 12 of the wafer is not touched by the injected solution 32 each time the wafer rotates. As a result, coating material 18 in the form of a bead remains on wafer 10.
The second cleaning process is a solvent backside rinse such as that shown in FIG. 3. The backside 48 of wafer 10 can be cleaned by this process. A cleaning solution 52 is injected from a spray nozzle 54 onto the backside 48 of the wafer. The process is also known as a centrifugal spray cleaning process wherein a chemical solution, i.e., normally a good solvent for the coating layer is pressure-fed and injected directly onto the backside of a spinning wafer. The process can be effectively used to reduce the volume of fresh chemical consumed and is normally faster than an immersion process. After the injected chemical solution 52 hits the bottom surface 48 of the wafer, the chemical solution 56 reflects from the backside 48 of the wafer and drains into the bottom of the process tank (not shown). During a normal backside rinse process, the sprayed chemical solution 52 is only capable of rinsing the backside 48 of the wafer and, none of the chemical solution 52 can reach the top surface 16. The bead 18 at the flat side 12 of the wafer is therefore not affected or cleaned in the backside rinse process.
Consequently, the SOG bead remains on the flat side of the wafer and eventually leads to SOG cracking during subsequent processes when a wafer clamp is pressed down on the SOG bead for mounting the wafer. The particles generated by the cracking of the SOG layer contaminate the surface of the wafer and is detrimental to the yield and the quality of the IC devices produced.
It is therefore an object of the present invention to provide a method for removing a coating layer from an unintended area of a wafer that does not have the drawbacks or shortcomings of the conventional cleaning methods.
It is another object of the present invention to provide a method for removing a coating layer from wafer flat side to eliminate the formation of a bead of the coating material.
It is a further object of the present invention to provide a method for removing a coating layer from an unintended area on the wafer flat edge such that the wafer can be processed in subsequent processes without producing particle contaminants.
It is another further object of the present invention to provide a method for removing a spin-on-glass material from a wafer flat edge such that a SOG build up at the flat edge can be avoided.
It is still another object of the present invention to provide a method for removing a SOG layer from wafer flat edge by utilizing a wafer immersion technique.
It is yet another object of the present invention to provide a method for removing a SOG coating layer from wafer flat edge by immersing the wafer in a solvent.
It is still another further object of the present invention to provide a method for removing a coating material from wafer flat edge by utilizing a solvent immersion method wherein a mixture of solvents is used depending on the coated material to be removed.